The Speed of Light

August 16, 2013

Everyone hates speed limits. I understand and tolerate the 65 mile-per-hour speed limit on most US interstate highways, since it allows a measure of safety against driver reaction time. What I don't tolerate are those stretches of local road where I'm sure the artificially low speed limit was set merely to generate revenue.

The Danishastronomer, Ole Rømer, was the first to actually affix a quantity to the speed of light. He did this by noticing that the period of Jupiter'smoon, Io, was shorter when the Earth was approaching Jupiter, and longer when receding from it. Rømer's estimate was 220,000 kilometers/sec, about 75% of the established value. A more accurate astronomical measurement was made in 1729 by English astronomer, James Bradley, who used his discovery of the aberration of light to calculate its speed to within 1.5% of its established value.

Since 1983, we are privileged to know the exact value of the speed of light. This value, 299,792.458 kilometers/sec was fixed by a new definition of the meter. Now, any improvement in the measurement of the speed of light is actually an improvement in the definition of the meter. That being said, everyone still uses the 300,000 kilometers/sec value. As my building contractor father would say when surveying for the foundation of a house, "It's close enough for digging."

Since the speed of light is a fundamental quantity and a part of the equations of so many theories, it's still studied. Earlier this year I reported on the possibility that the speed of light we measure is an average value. It might show variation over very short length scales (Light Speed, April 8, 2013).

The experiment studied the energy required to change the velocity of electrons in transitions from one atomic orbital to another in dysprosium. Dysprosium has a pair of orbitals very close in energy for which the electrons travel at different speeds, so small changes in the electron kinetic energy can be detected.[5] The electron orbitals of the atoms were partially aligned by the polarization of the exciting laser beams, so the rotation of the Earth was equivalent to sampling the electron velocity in different directions of space.[5] The measurements were an order of magnitude more sensitive than previous studies.[4-5]

The dysprosium atoms functioned as atomic clocks having a relative frequency difference. It was found that in a two year period in which the Earth's distance to the Sun changed, the relative frequency of the clocks was constant. This finding was consistent with the local position invariance of general relativity that the kinetic energy of an object is independent of its location in a gravitational field.[4-5]

Berkeley physics postdoc, Michael Hohensee, summarized his motivation for such experiments by saying,

"As a physicist, I want to know how the world works, and right now our best models of how the world works – the Standard Model of particle physics and Einstein's theory of general relativity – don't fit together at high energies... By finding points of breakage in the models, we can start to improve these theories."[4]

where α is the dimensionless fine structure constant. So, if α changes, is it because e, h or c changes? Any such changes would be notable.

The research team was able to adapt their experiment to perform the same measurements for the fine structure constant. Their results, accepted for publication in Physical Review Letters and available online at arXiv,[6] show that the fine structure constant does not vary with time or gravitational field.[5-6] Their result for the time change of α was (1/α)(∂α/∂t) = (-5.8±6.9)×10-17 yr-1, consistent with zero.[6]

The experimental results, although impressive, were not optimized. The present order of magnitude improvement over past measurements is expected to reach three orders of magnitude in the future.[2-3,5] If there's some secret hiding in c or α, it might be found in the next few years. This research was funded by the National Science Foundation, the Australian Research Council, and other agencies.[4]